During their lifetime, proteins inevitably expose hydrophobic segments within the polypeptide chains on a molecule's surface, which may be otherwise buried inside the molecules in the proper conformation. This potentially dangerous situation is managed with the aid of the 70-kDa heat shock proteins (Hsp70s) and other molecular chaperones. Although a major function of Hsp70 is assisting in efficient folding of anonymous proteins in unfolded states, recent studies have revealed that Hsp70 plays a variety of specific roles, sometimes deciding the cell fate. These multiple activities are based on the specific binding of Hsp70 to proteins in native states, which regulate cell growth and/or death. It is now well recognized that unfolding of some proteins may cause serious diseases, especially those associated with neurodegeneration, such as Alzheimer's disease. It is suggested that Hsp70 might be a potential drug against these diseases, but caution should be taken because Hsp70 exerts multiple effects by binding to specific proteins.